(query-replace-read-args): Implement `\,' and `\#'

(query-replace-read-args): Implement `\,' and `\#'
replacements here.
(query-replace-regexp): Doc string explaining this and the new
`\?' replacement.  Remove `\,' and `\#' implementation here, as it
is better placed in `query-replace-read-args'.
(replace-regexp): Explain `\,', `\#' and `\?'.
(replace-match-data): New function for thorough reuse/destruction
of old match-data.
(replace-match-maybe-edit): Function for implementing `\?'
editing.
(perform-replace): Fix maintaining of the match stack including
already matched regions, implement `\?', fix various problems
with regions while editing and other stuff.
(replace-highlight): Simplified.